1. Field of the Invention
This invention is directed to methods and apparatuses for quantitative determinations of the strengths, pore pressures and mechanical properties of low permeability geologic materials, particularly low permeability rock, and, in one aspect, to the measurement of shale strength, including the strengths of hard, illitic sloughing shales and soft, smectitic gumbo shales under varying compaction and water content conditions.
2. Description of Related Art
Subsurface formations encountered in oil and gas drilling are compacted under in situ stresses due to overburden weight, tectonic effects, confinement and pore pressure. Shales make up over 75% of drilled formations and cause over 90% of the wellbore stability problems. When a hole is drilled in a formation, the wellbore rock is subjected to increased shear stresses due to a reduction in confinement at the wellbore face by removal of the rock from the hole.
Compressive failure of the rock near the wellbore will occur if the rock does not have sufficient strength to support the increased shear stresses imposed upon it. However, if the hole is filled with drilling fluid with sufficient density to increase the wellbore pressure or confining pressure to a proper level, the shear stresses imposed on the wellbore rock will be reduced and the hole will remain stable. If the wellbore pressure is increased too much, lost circulation or hydraulic fracturing of the formation will occur as a result of tensile failure of the wellbore rock.
Previous workers have applied classical elastic and elasto-plastic theories, failure criteria and fracture mechanics to model wellbore behavior under different stress conditions. In 1979 an elastic model was used with field data to obtain stress and pore pressure and empirically derived rock strength values to predict wellbore behavior. In 1983, the present inventor and others modeled the behavior of high angle wells at Statfjord Field in the North Sea by using classical theories and field data to obtain stress, pore pressure and empirically derived shale strength data from several of the early wells. The results of that study were published in a report that was widely disseminated throughout the petroleum and rock mechanics communities. The model results provided wellbore stability charts that showed the proper wellbore pressures or equivalent drilling fluid weights or densities required to prevent wellbore collapse and lost circulation as a function of hole angle and depth. The wellbore stability charts were used as engineering guidelines for subsequent wells and saved many millions of dollars by reducing trouble costs due to wellbore failure. This case and subsequent cases showed that if shale strength data were available, a wellbore model could be used on initial wells in an area to diagnose and predict wellbore behavior and to prevent expensive stuck pipe problems and high trouble costs. Others recognized the importance of this approach and emphasized the need for shale strength data. Physical measurements of shale properties are required to develop realistic failure criteria (shale strengths) and constitutive relationships to effectively understand and define shale behavior under downhole conditions.
The term "shale" includes extremely low permeability clay-bearing rocks. Shales are extremely difficult to test for several reasons. They contain hydratable clays that make them water sensitive and cause them to take up water and swell when exposed to water. Uncontrolled hydration or drying can cause rapid deterioration of the rock structure making it difficult to obtain and maintain sample integrity. Water contents of the shales can be quite variable. In addition, shales have extremely low permeabilities that are in the microDarcy to nanoDarcy range. Permeability is the ease with which fluids can move between voids in a material. Permeability is defined as the capacity of a formation to transmit fluids with the unit of measurement thereof being the "Darcy". One Darcy is defined as that permeability permitting a fluid of one centipoise viscosity to flow at a rate of one cubic centimeter per second through a cross-sectional area of a square centimeter when the pressure gradient is one atmosphere per centimeter. The formula generally used to determine rock permeability is: ##EQU1## where: K=permeability in Darcys
Q=quantity of flow PA1 U=viscosity of fluid PA1 A=area of cross-section of core PA1 L=length of core PA1 P=applied fluid pressure. PA1 "Methods For Determining In Situ Shale Strengths, Elastic Properties, Pore Pressures, Formation Stresses And Drilling Fluid Parameters" naming Messrs. Ronald P. Steiger and Peter K. Leung as co-inventors. PA1 "Test Apparatuses And Methods For Adjusting A Material's Fluid Content And Effective Stresses," naming Messrs. Ronald P. Steiger and Peter K. Leung as co-inventors. PA1 "Apparatuses and Methods For Measuring Ultrasonic Velocities In Materials," naming Messrs. Ronald P. Steiger and Peter K. Leung as co-inventors. PA1 "Microaccumulator For Measurement Of Fluid Volume Changes Under Pressure," naming Messrs. Ronald P. Steiger, Peter K. Leung, and Rudolf J. Stankovich as co-inventors.
The results of this equation is in milliDarcys (1/1000 of a Darcy.)
Using standard routine prior art methods and equipment, measured values range from as low as 0.1 milliDarcy to as high as 20,000 milliDarcys. Special prior art pressure-pulse techniques can measure values in the nanoDarcy range (about one billionth of a Darcy). Shales are usually referred to as impermeable rocks and thus have been precluded from direct triaxial testing. Much triaxial strength testing has been done on higher permeability rocks, such as sandstones, clays and soils. Sandstones have permeabilities to water in the Darcy to milliDarcy range; clays and soils in the Darcy to microDarcy range. For example, consolidated sodium montmorillonite clay, one of the least permeable clays, has a permeability of about 10 microDarcys.
The prior art related to the more permeable rocks and soils teaches that the shear strength of the rock generally varies as a function of the mean effective stress (or effective confining stress). Shear strength or the shear stress at failure versus mean effective stress for the porous rock is called its failure criterion or constitutive relationship for failure. The principle of effective stress is recognized as the keystone of modern soil mechanics and was developed by Terzaghi ["Soil Classification for Foundation Purposes," Trans. 1st Int. Cong. Soil Sci., Washington D.C., 1927, Deemer, R.B., Ed., 1928, 4, 127]. The principle states that the stress at any point in a saturated soil mass can be described by the effective stress, .sigma.', which represents the intergranular stress between solids of the soil or rock or the difference between the total stress, .sigma., and the pore pressure, u, as follows: EQU .sigma.'=.sigma.-u
According to the effective stress concept, all the measurable effects of a change of stress, such as compression and change of shearing resistance are exclusively due to changes in the effective stresses. In the simplest terms, the effective stress principle asserts that the effective stress controls volume change and strength. The significance of the effective stress concept to soil mechanics is that macroscopic deformation behavior is governed by effective rather than total stresses.
The recognition of the effective stress principle had provided a valuable tool to explain soil and rock behavior and has led to the development of many useful failure criteria, unified soil constitutive relationships, such as critical state soil mechanics, [Atkinson, J. H. and Bransby, P. K., The Mechanics of Soils, McGraw-Hill (UK), London (1978) 184-234] and stress path analysis [Lambe, T. W., "Stress Path Method," ASCE, JSMFD, 93, SM6 (1967) 309-331]. However, there was (e.g. in 1960) a general belief among some researchers that the effective principle is not valid for shales. That is because shale permeabilities are so extremely low that their pore spaces are ill-defined and possibly not adequately interconnected to provide pore pressure equilibrium or direct measurement. Some believe that pore pressures can not be measured or defined in shales.
It has been a point of controversy as to whether or not low permeability shales (often called impermeable shales) also follow an effective stress relationship. Limited experimental evidence has been previously obtained by indirect methods that indicate that they may. In 1982, Tosaya ["Acoustical Properties of Clay Bearing Rocks," Ph.D. Thesis, Stanford University (1982)] demonstrated indirectly the effective stress principle for shales. In her experiment, acoustic velocities in isostatically loaded shale samples were found to correlate with effective stresses, but not total stresses. The effective stresses were derived from long-term experiments in which pore pressure was externally applied via a line connected directly to the sample and an isostatic total stress applied to the outside of the jacketed sample. Then acoustic velocities were measured on the equilibrated sample. Several such long-term experiments produced excellent correlations with effective stress. However accurate, direct pore-pressure measurements on low permeability shales during triaxial tests have not been made on an equilibrium basis. Standard triaxial test procedures used for higher permeability rocks and soils have not produced good results when applied to low permeability shales.
Conventional tests for measuring properties of rocks, under a triaxial state of stress in which stresses act simultaneously along three orthogonal axes of a specimen, are often conducted in a pressure vessel called a triaxial test cell that contains the specimen and a confining fluid and receives a piston to extend into the vessel. The test cell is typically sufficiently large and open to admit the specimen and confining fluid. A hydraulic pump compresses the confining fluid surrounding the specimen, applying an all-around hydrostatic or isostatic stress to the specimen. An axial loading force is applied to the loading piston that moves into the test cell applying a deviatoric axial (triaxial) stress to the test specimen which is greater than a hydrostatic stress exerted on the sample by the pressurized fluid.
In a conventional triaxial test apparatus for rock testing, the sample is deformed by gradually increasing the axial load until the peak strength of the specimen is reached at which point it fails; i.e., it will not sustain any further increased axial loading. The pore pressure within the sample is measured by a pressure gauge or transducer outside the test cell that is attached to a pore pressure line that runs into the test cell through the end cap to the rock face. Due to the relatively high permeability of the rock, pore pressure equilibrium within the rock and at the pressure gauge or transducer can be achieved rather quickly. Thus the pore pressure response can be fast and monitored easily. Alternatively, pore pressure can be applied and controlled externally during a test by pumping water into the sample through the pore pressure line. The loading stage of the conventional test can be relatively fast and typically at high strain rates in the range of 10.sup.-2 to 10.sup.-4 sec.sup.-1 and sometimes as slow as 10.sup.-5 to 10.sup.-6 sec.sup.-1 and still achieve pore-pressure equilibrium during the test. Additionally, highly accurate control of the strain rates is not critical and usually not achieved for the typical high strain rates used for higher permeability rocks. An LVDT (linear variable differential transformer) on the load piston is usually used to control the strain rate, however more recent applications have used LVDT's on the rock or end caps to obtain more precise control.
In our prior art efforts to measure shale strengths, many triaxial tests were performed on several shale types with direct pore-pressure measurements under equilibrium conditions to demonstrate directly that shales follow the general effective stress relationship. This technical achievement preceded work in generalizing the mechanical behavior of shales using effective stress methods. Thus, if the relationship between effective stress and strength, compressibility or expansability is determined experimentally, then the in situ behavior of shales can be predicted. These efforts are described in "Quantitative Determination of the Mechanical Properties of Shales," Paper presented at the 63rd Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Oct. 2-5, 1988.
As our prior art teaches, it is known to cut water-sensitive shale test specimens from undisturbed block samples under controlled humidity conditions with an inert coring fluid to prevent damage. Sample size is chosen to optimize water content equilibrium and pore pressure equilibrium. The water contents of the shale test specimens are adjusted under equilibrium conditions in a humidity chamber to obtain predetermined water contents without causing damage to the rock. The size of the sample is chosen to minimize the time period required to reach an equilibrium water content, yet large enough to be representative of the rock tested. All subsequent handling of the core prior to testing is done in a controlled humidity chamber to preserve the core. The shale test specimens are in the shape of a small core (right circular cylindrical core). The sample size is chosen also to allow pore pressure equilibrium to be established throughout the sample and with the pore pressure transducer while loading the rock at strain rates that will allow the tests to be conducted in a reasonably short time period, i.e. within about 1-3 days. Thus actual equilibrium pore pressure values can be measured continuously throughout the triaxial test. The pore pressure is measured in the tests by a triaxial end cap that incorporates an accurate miniaturized pressure transducer into the end cap near the rock face. Additionally, it obviates external contact with water from a pore pressure line and thus preserves the original water content of the test specimen throughout the test. It is a stiff assembly that provides accurate pore pressure measurements and maintains volumetric control over the water in the test specimen. The diameters of the end caps are matched to the diameter of the test specimens. The end caps typically have flow channels or grooves therearound and thereacross on the face of the end cap connected to the pore pressure line opening. The small-volume channel between the rock face and the pore pressure transducer is filled with an inert fluid to provide direct pressure transmission and to prevent water drainage from the rock. The sample core is mounted between the end caps and sealed in an impermeable flexible jacket. The sample core mounted between the end caps is instrumented to measure accurate axial and orthogonal transverse strains during the tests by a computerized data acquisition system. Various such systems are available. A conventional heavy duty, high load capacity triaxial test load from with a high pressure confinement vessel (or cell) is used. The system is outfitted with a computer control and data acquisition system. A computer program provides very accurate data acquisition and control of the triaxial test equipment during a test. Precise computer-controlled axial loading of the test specimen is accomplished by a feedback loop that is based on actual strain measurements on the rock. "Strain" is defined as the compression (positive) or extension (negative) resulting from the application of external stresses divided by the original dimension; e.g. axial strain is the fractional change in length of the core specimen The computer control system provides continuous, accurate, strain rates from 10.sup.-2 to less than 10.sup.-8 sec.sup.-1. This feature provides a capability to simulate extremely stiff load frame characteristics and allows complete stress-strain measurements past peak failure to residual or ultimate stresses to be obtained. It also provides the capability to load the rock at a rate slow enough to obtain equilibrium pore pressure, stress and strain measurements throughout the test.
There has long been an need for methods and apparatuses for measuring the strengths and mechanical properties, including (but not limited to) pore pressure, geologic materials, including but not limited to, low permeability rock, including shales. There has long been a need for a test in which the effective stresses of a low permeability rock can be accurately measured during the test. There has long been a need for a test apparatus and method with which sample equilibrium can be achieved in a practical amount of time. There has long been a need for methods and apparatuses for making quantitative determinations of the strengths and mechanical properties of low permeability rock with which air, which could adversely affect the test results, is not entrapped in the apparatus. There has long been a need for such a test in which fluids are employed which are not miscible with water or with the pore fluid will not wet or react with the test specimen. Also there has long been a need for a test in which the mean effective stress of a low permeability rock can be controlled by adjusting the amount of water in the sample before the test and a test in which mean effective stress can be varied by adjusting the amount of water in the sample prior to testing. There has long been a need for such tests in which a minimal amount of pore fluid is expelled from a sample during testing so that the sample's water content is maintained almost constant while pore pressure is measured directly.